Novel approaches to epilepsy therapy are investigated using animal seizure models. In the present reporting period, the evaluation of the role of neuroactive steroids in epilepsy and their possible use as anticonvulsants was continued. Neuroactive steroids are endogenous metabolites of certain steroid hormones (and their synthetic analogs) that rapidly alter the excitability of neurons by direct actions on membrane ion channels. In a prior reporting period, several structurally-related neuroactive steroids were demonstrated to be potent anticonvulsants in the mouse pentylenetetrazol (PTZ) seizure test, an effect that is likely due to their ability to potentiate GABA-evoked chloride currents. In addition, it was shown that GABA-potentiating neuroactive steroids are highly protective in models of status epilepticus. Finally, chronic treatment studies indicated that, in contrast to other GABA potentiating drugs, tolerance does not develop to the anticonvulsant activity of neuroactive steroids. In the present reporting period, we sought to determine whether the anticonvulsant activity of the sex hormone progesterone is mediated via endogenous conversion to the neuroactive steroid epiallopregnanolone (3- alpha,5-alpha-P). Progesterone is believed to be converted to 3-alpha,5- alpha-P via reduction by steroid 5-alpha-reductase, a system of two NADPH- dependent isozymes. We used the azasteroid finasteride, a competitive inhibitor of both forms of 5-alpha-reductase, to assess whether the anticonvulsant activity of progesterone is dependent upon enzymatic reduction. Progesterone protected mice against pentylenetetrazol-induced seizures in a dose-dependent manner. Coadministration of finasteride reversed the protective effect of progesterone. In contrast, finasteride failed to affect the anticonvulsant activity of 3-alpha,5-alpha-P. These studies provide strong evidence that the anticonvulsant activity of progesterone is mediated by its active metabolite 3-alpha,5-alpha-P. The studies further suggest that changes in seizure susceptibility associated with the menstrual cycle or during pregnancy where there are large fluctuations in progesterone levels could be due to corresponding changes in the levels of the anticonvulsant metabolite 3-alpha,5-alpha-P.